1. Field of the Invention
The disclosed and claimed concept relates to a system utilizing a nozzle, which is closed by an internal needle, structured to dispense liquids and, more specifically, to a system for calibrating the needle lift relative to the nozzle body.
2. Background Information
Certain fluid dispensing systems are structured to dispense a liquid that is less viscous when hot. If the liquid is allowed to cool, the viscosity increases making it more difficult to apply the liquid in a controlled and/or consistent manner. Such liquid dispensing systems may utilize nozzle assemblies, or “spray guns” that are closed by an internal needle. The liquid may be a sealant or an adhesive. The remainder of this description shall use an adhesive as an example, but it is understood that the liquid is not limited to an adhesive. Generally, an adhesive is either a solvent based adhesive or a water based adhesive. In some aspects, the spray gun is adapted to a specific type of adhesive. For example, a solvent based system will include a temperature control to maintain the temperature of the liquid. The spray gun includes a housing that defines a chamber with a nozzle. The chamber includes a liquid inlet and may contain a liquid outlet. The liquid flows into the chamber via the liquid inlet. The liquid may be stored, briefly, in the chamber before application. For a water-based adhesive, the liquid is, typically, expelled exclusively via the nozzle. For a solvent based adhesive, a portion of the liquid may be dispensed via the nozzle and any excess liquid may be recycled exits the chamber via the outlet. The liquid may then be drained from the system, or, reheated and re-circulated. In this configuration, the liquid in the chamber may be maintained at a temperature that allows for a known, consistent flow rate. Typically, the liquids dispensed by such spray guns must be maintained in a very limited temperature range while in the spray gun liquid chamber.
The nozzle defines an internal passage having a generally frusto-conical shape, i.e. a frustum. The nozzle further includes an internal seat; the seat may be part of the internal surface of the nozzle. A needle having its longitudinal axis aligned with the axis of the nozzle passage is used to seal the passage. That is, the needle coupled to an actuator structured to move the needle in an axial direction; i.e. longitudinally. The needle proximal end is coupled to the actuator and the opposite end of the needle distal tip is shaped generally, or substantially, to correspond to the shape of the nozzle seat. When the needle is in a forward, first position, the needle distal tip sealingly engages the nozzle seat. In this configuration, the spray gun is closed. When the needle is in a retracted, second position, the needle distal tip is fully spaced from the nozzle seat. In this configuration, the spray gun is open. The distance between the needle and the seat is identified as the “needle lift.” Further, and as described below, the needle may also be placed anywhere between the first and second position, thereby causing the nozzle to be partially open. That is, when the needle is in the second position, i.e. fully spaced from the nozzle internal passage, the nozzle is, essentially, unblocked and allows for the nozzle's maximum flow rate. It is noted that, while in the second position, the needle may be disposed within the nozzle internal passage, so long as the nozzle achieves its maximum intended flow rate. If the needle is somewhere between the first and second positions, the nozzle is partially open and the liquid flows at a rate less than the maximum flow rate.
Typically, such spray guns must be opened and closed both rapidly and intermittently. That is, the nozzle is cyclically opened a brief period of time, then closed for a brief period of time. This would allow, for example, a quantity of sealant to be applied to an object while the spray gun is open, then for the object to be moved and replaced while the spray gun is closed. This is useful for an automated process or assembly line wherein objects such as, but not limited to, cans are moved through the fluid dispensing system.
Many nozzle assemblies of this design utilize a solenoid to move the needle between the first and second positions. There are at least two problems with such solenoids. First, the solenoids are disposed relatively close to the nozzles. This is a problem because the current that causes the rapid opening and closing of the spray gun also causes the solenoid to heat up. Because the solenoid is close to the spray gun liquid chamber, the liquid in the chamber may become heated. Further, changes in ambient temperatures may vary greatly. As noted above, the liquids dispensed by such spray guns must be maintained in a very limited temperature range while in the spray gun liquid chamber. Thus, the heat added to the liquid by the solenoid may raise the liquid above the desired temperature. Further, such solenoids typically have only two configurations; when the solenoid is charged, the needle is placed in the second, fully open position. When the solenoid is not charged, a spring or a similar device returns the needle to the first, closed position. Thus, there was no means to allow for a partial flow of the liquid.
As shown in U.S. Pat. No. 5,945,160, this later disadvantage was addressed by controlling the needle solenoid with two stepping solenoids, an opening stepping solenoid and a closing stepping solenoid. The stepping solenoid rod was coupled to the needle solenoid and moved the needle solenoid forward and aft in the spray gun housing. Rather than using a charged coil, and possibly a spring, to move a rod forward and back, a stepping solenoid uses a charge to incrementally move a rod in one direction. That is, each incremental movement was a “step.” The incremental motion may be achieved by rotating a rod disposed in a threaded bore as opposed to moving the rod axially. That is, a stepping solenoid may include a fixed threaded bore and the solenoid rod may have a threaded portion engaged therewith. Actuation of the stepping solenoid coil causes the solenoid rod to rotate a portion of a revolution, i.e. the increment noted above. This rotation causes the solenoid rod to move axially relative to the fixed threaded bore. Thus, the solenoid rod may be moved incrementally axially. That is, a single actuation of the stepping solenoid coil causes the solenoid rod to rotate over an arc, e.g. 5 degrees. Multiple actuations therefore cause the solenoid rod to move over multiple arcs, in this example, arcs of 5 degrees each. Each partial rotation of the rod moves the rod axially relative to the threaded bore. Thus, the stepping solenoid rod may be “stepped” forward. Use of a second stepping solenoid allows for movement in the other direction, i.e. the solenoid rod may be stepped backward. As such, the position of the needle solenoid in the spray gun housing, and therefore the position of the needle, could be adjusted.
Such stepping solenoids are useful as the needle lift may be changed during the use of the spray gun. For example, assume a spray gun is inactive at night. When the spray gun is first used in the morning, it is cold and the heated liquid being applied by the spray gun is not affected by the spray gun temperature. With the liquid at this temperature, the needle lift should be 0.035 inch. As the day goes on, the ambient temperature increases, thereby raising the temperature of the liquid. By noon, the stored liquid is less viscous and, to achieve the same results as in the morning, the needle lift needs to be reduced to 0.015 inch. This type of adjustment cannot be accomplished with traditional, non-stepping solenoids. A system having a stepping solenoid can make such an adjustment.
The stepping solenoids typically respond to an input in the form of a pulse. That is, the stepping solenoids are energized, and therefore move one increment, in response to receiving a single pulse of energy. This energy may be supplied directly to the solenoid coil, or may be used to open and close a circuit that energizes the solenoid coil. For each pulse received, the solenoid moves one increment or step. Thus, to move the solenoid rod and needle a selected distance, e.g. 0.015 inch, the stepping solenoid would have to receive thirty pulses. The actuator control system records the number of pulses sent to each stepping solenoid thereby tracking the position of the needle.
While this type of spray gun allowed for greater control of the liquid flow rate, i.e. by allowing the needle to be placed in multiple partially open configurations, another problem developed; the needle was not always where the actuator control system “believed” it to be. That is, the actuator control system's record of where the needle was positioned was not always accurate. The actuator control system includes a memory and a processor, hereinafter a programmable logic circuit (PLC), or similar device structured to execute a series of instructions. The memory includes the instructions for the PLC, typically stored in “modules,” as well as data stored in a register. Some of the stored data includes data representing the “virtual” needle position. That is, a virtual needle position module correlates the change in needle position with the recorded number of pulses sent to each stepping solenoid. Alternatively, the data could be compiled into a “virtual” needle position database that correlates a number of pulses with a virtual needle position, e.g.:
Number ofNeedle Lift in InchesPulses(Virtual Needle Position)10.000520.001030.001540.002050.0025Thus, rather than calculating the needle position, the virtual needle position module could just record the type (i.e. forward or backward motion) and number of pulses sent and then look up the corresponding virtual needle position.
For example, assume the virtual needle position module correlates each pulse with a needle lift of 0.0005 in. Further assume the needle starts in the first position. If the opening stepping solenoid was sent thirty pulses and the closing stepping solenoid 100A was sent fifteen pulses, the needle position module would record the needle as having a needle lift of 0.0075 in. That is, a movement of 0.0005*30=0.015 in. away from the needle seat and 0.0005*15=0.0075 toward the needle seat totals a needle lift of 0.0075 in. The recorded needle position is identified as “virtual” as the actuator control system cannot verify that the needle is actually 0.0075 in from the nozzle seat. In fact, many times the needle does not have a needle lift matching the “virtual” needle position.
This offset between the virtual needle position and the actual needle position was caused by various factors. For example, the manufacturing tolerances of the various spray gun components is in the range of about +/−0.0001 to +/−0.0005 inch. Thus, during assembly of the gun, the stacking of tolerances may create an error as to the actual position of the needle. Further, the initial measurement of needle lift was performed manually, which could also be in error. For example, the operator could make an error in the actual measurement, an error in entering the manual measurement data, or may even forget to enter the data into the control PLC. Additionally, the gun may, for many reasons, fail to operate, e.g. advance or retract the needle, in a controlled manner. Thus, either from the initial usage or developing over time, the virtual needle position and the actual needle position may not be the same. This is a disadvantage as movement of the needle is based upon the data representing the virtual needle position.
Manufacturers of spray guns recommend that regular maintenance be performed on the spray gun nozzles to recalibrate the needle lift. This operation typically requires that the spray gun be taken offline so that the actual needle lift may be measured. This data is then incorporated into the virtual needle position database. This procedure is time consuming, requires an expensive external calibration device, and requires cleaning the spray gun to remove the sealant. As such, users have been known to recalibrate a spray gun by unapproved methods such as forcing the needle forward and resetting the virtual position. That is, the users actuate the closing stepping solenoid for a period causing the needle to move forward. After a short, but indeterminate period of time, the needle engages the seat. As the user does not know when this occurs exactly, the user typically allows the closing stepping solenoid to continue operating. This continued forward motion of the needle may cause damage to the needle, the nozzle seat, and the closing stepping solenoid. Once, the user stops the closing stepping solenoid, the virtual needle position database is updated to indicate that the current needle position is the first position.